1. Field of Invention
The invention relates to an interleaver structure and, in particular, to an interleaver used in a high-density wavelength division multiplexing fiber communication systems to achieve wavelength division multiplexing and de-multiplexing.
2. Related Art
The basic structure of a high-density wavelength division multiplexing optical fiber communication system is to divide mostly used transmission spectra in the optical fiber into several transmission channels according to the lightwave frequencies. Each channel uses its central frequency as its carrier frequency (such as ITU 100 GHz grid). The signals in all channels are merged together to be transmitted over one fiber so as to increase its transmission capacity. This is called the wavelength division multiplexing (WDM). Afterwards, the signals of all channels transmitted in the fiber are separated into different fibers to perform signal processing. This is called the wavelength division de-multiplexing (WDDM). Taking a wavelength division de-multiplexing system with a frequency interval of 50 GHz as an example (FIG. 1), there are optical signals satisfying the ITU grid 50 GHzxc3x9780 channels transmitting in the fiber. A 50 GHzxc3x9780 channels wavelength division de-multiplexer (DEMUX) can de-multiplex the signal light of all wavelength channels so that the signal of each channel is guided into a distinct fiber.
Alternatively, an interleaver can be equipped with two few-channel and low-density channel spacing multi-channel de-multiplexer to achieve the same object (FIG. 2). Similarly, optical signals satisfying the ITU Grid 50 GHzxc3x9780 channels are transmitted therein. In this case, a 50 GHz 1xc3x972 interleaver can be used to perform the first stage de-multiplexing on the signal light. According to the functioning principles of interleavers, optical signals in 40 odd channels are output from one of the output terminal. Such optical signals satisfy the ITU Grid 100 GHz wavelength channel standard. Meanwhile, optical signals of the other 40 even channels are output from another output terminal. Such optical signals satisfy the ITU Grid 100 GHz wavelength channel standard with a channel offset of 50 GHz. Both of the optical signals can be de-multiplexed using 100 GHz multi-channel de-multiplexers (one of them having an offset of 50GHz) so that the optical signals of each channel are transmitted into distinct optical fibers.
Although the above two methods perform the same function, the later has a lower cost because the manufacturing of the 50 GHzxc3x9780 channels DEMUXs is much harder than that of the 100 GHzxc3x9740 channels DEMUXs. Also, the channel number of a single device is twice that of a 100GHz one, therefore the cost is often several times more expensive than that of two 100 GHz DEMUXs. Thus, using the structure in FIG. 2 costs much less than that of FIG. 1 even if an additional interleaver is included.
From the viewpoint of system upgrading, interleavers are indispensable. Suppose a 100 GHzxc3x9740 channels DEMUX transmission system needs to be upgraded into a 50 GHzxc3x9780 channels DEMUX system to double the transmission capacity. If one considers to upgrade it to the structure shown in FIG. 1, the original 100 GHz DEMUX has to be replaced by a 50 GHzxc3x9780 channels DEMUX. Besides the cost increases, the abandoned device is also a waste of cost. If one wants to upgrade it into the structure shown in FIG. 2, the 100 GHz DEMUX can be kept. One only needs to add another 100 GHz DEMUX (with an offset of 50GHz) and an interleaver. It does not increase or waste extra cost.
An object of the invention is to provide a planar lightwave circuit (PLC) interleaver that mainly utilizes a PLC structure.
Along with several few-channel and low-density wavelength division multiplexer/de-multiplexer, a novel planar lightwave circuit (PLC) interleaver is invented to achieve high-density wavelength division multiplexing and de-multiplexing in a wavelength division multiplexing fiber communication system.
The invention uses a PLC structure as its technical basis. Utilizing the principle that the product of the number of output optical waveguides and the channel spacing is equal to the free spectral range of any output optical waveguide spectrum, a PLC interleaver structure is designed to, along with several few-channel and low-density wavelength division multiplexer/de-multiplexer, construct a high-density WDM fiber communication system.
The disclosed PLC interleaver can be integrated with other PLC wavelength division multiplexers/de-multiplexers or other complicated PLC devices on the same chip through the PLC manufacturing techniques. The product does not only feature in small volume but is also convenient for mass production at a lower cost.